Some image-forming devices employ a belt unit for conveying a recording medium on which images are formed. This belt unit generally includes a drive roller and a follow roller, and a conveying belt looped around the drive roller and follow roller. However, it is difficult to form conveying belts with a uniform thickness over the entire length and, consequently, the belt units must employ conveying belts having a variable thickness component.
However, in a structure that transfers the driving force of the drive roller unchanged to the follow roller via a conveying belt having such inherent variations, the variations in the thickness component will translate to variations in the driving speed transferred to the follow roller. These variations in driving speed will cause the recording medium to be conveyed at an uneven speed.
When such a belt unit is incorporated in an image-forming device having image-forming units disposed along a conveying direction, the irregular conveying speed will shift the position of images recorded by individual image-forming units in the conveying direction, leading to a decline in the quality of the overall image formed on the recording medium. This decline in image quality is more pronounced as the distance between the image-forming units becomes great.
Currently, technologies have been proposed for preventing this drop in image quality. For example, Japanese unexamined patent application publication No. 2003-173067 proposes a technique for controlling the rotational speed of a driving motor that drives the drive roller to rotate based on variations in the thickness component of the conveying belt (variations in thickness with respect to the length of the belt).
However, this technology requires changing parameters of the belt unit for conveying the recording medium, such as the rotational speed of the drive roller, in real time in response to changes in the thickness component of the conveying belt. Therefore, a great process load is placed on controlling the driving motor in order to change these parameters.